Light Cycle Oil (hereinafter, called “LCO”) as cracked light oil that is generated by a fluidized catalytic cracking contains a large amount of polycyclic aromatic hydrocarbon and is used as light oil or heavy oil. However, in recent years, investigations have been conducted to obtain, from LCO, monocyclic aromatic hydrocarbons having 6 to 8 carbon numbers (for example, benzene, toluene, xylene, ethylbenzene and the like), which can be utilized as high octane value gasoline base materials or petrochemical feedstocks and have a high added value.
For example, Patent Documents 1 to 3 suggest methods for producing monocyclic aromatic hydrocarbons from polycyclic aromatic hydrocarbons that are contained in LCO and the like in a large amount, by using a zeolite catalyst.
However, Patent Documents 1 to 3 do not disclose that the yield of monocyclic aromatic hydrocarbon having 6 to 8 carbon number produced by the method is sufficiently high.
When monocyclic aromatic hydrocarbons are produced from heavy crude oil containing polycyclic aromatic hydrocarbons, catalyst regeneration for removing a carbonaceous substance needs to be performed with a high frequency since a large amount of carbonaceous substance is precipitated on the catalyst and rapidly decreases the activity. Moreover, when a circulating fluidized bed for performing a process of efficiently repeating reaction-catalyst regeneration is employed, the temperature for catalyst regeneration needs to be higher than the reaction temperature, so the temperature environment of the catalyst becomes more severe.
When a zeolite catalyst is used as a catalyst under such a severe condition, hydrothermal deterioration of the catalyst progresses, and the reaction activity decreases over time. Accordingly, the improvement of hydrothermal stability is required for the catalyst. However, for the zeolite catalyst disclosed in Patent Documents 1 to 3, a measure for improving hydrothermal stability was not taken, and the practical usefulness thereof was extremely low.
As the method for improving hydrothermal stability, a method using zeolite having a high Si/Al ratio, a method of stabilizing a catalyst by performing hydrothermal treatment in advance, such as in USY type zeolite, a method of adding phosphorus to zeolite, a method of adding a rare-earth metal to zeolite, a method of improving a structure directing agent at the time of zeolite synthesis, and the like are known.
Among these, addition of phosphorus is known to have effects that improve not only the hydrothermal stability but also the selectivity resulting from inhibiting the precipitation of a carbonaceous substance during fluidized catalytic cracking, the abrasion resistance of a binder, and the like. Accordingly, phosphorus is frequently added to catalysts for a catalytic cracking reaction.
Catalysts for catalytic cracking that are obtained by adding phosphorus to zeolite are disclosed in, for example, Patent Documents 4 to 6.
That is, Patent Document 4 discloses a method of producing olefins from naphtha by using a catalyst containing ZSM-5 to which phosphorus, gallium, germanium, or tin has been added. Patent Document 4 aims to improve the selectivity in generating olefins by inhibiting generation of methane or an aromatic fraction by method of adding phosphorus, and to improve the yield of olefins by securing high activity with a short contact time.
Patent Document 5 discloses a method of producing olefins from heavy hydrocarbons with a high yield, by using a catalyst in which phosphorus is supported on ZSM-5 containing zirconium and a rear-earth metal and a catalyst which contains USY zeolite, REY zeolite, kaolin, silica, and alumina.
Patent Document 6 discloses a method of producing ethylene and propylene with a high yield, by converting hydrocarbons by using a catalyst containing ZSM-5 supporting phosphorus and a transition metal.
As described above, addition of phosphorus to zeolite is disclosed in Patent Documents 4 to 6. However, all of the methods mainly aimed to improve the yield of olefins, and failed to produce monocyclic aromatic hydrocarbons having 6 to 8 carbon number with a high yield. For example, though Table 2 of Patent Document 6 discloses the yield of olefins (ethylene and propylene) and BTX (benzene, toluene, and xylene), the yield of olefins is 40 mass %, but the yield of BTX is as low as about 6 mass %.
Accordingly, a catalyst for producing monocyclic aromatic hydrocarbons that makes it possible to produce monocyclic aromatic hydrocarbons having 6 to 8 carbon number with a high yield from oil feedstock containing polycyclic aromatic hydrocarbons and to prevent the reduction in the yield of the monocyclic aromatic hydrocarbons caused over time has not practically become known.